Move port to depend on graphics/osg, which was recently updated to
3.2.0, and osgearth's requirement was 3.0.1, to avoid conflicts with
other ports that depend on graphics/osg.
Make the graphics/osg version a variable, to ease maintainer's jobs as
graphics/osg gets updated (which requires osgearth's PORTREVISION and
PLIST_SUB to be bumped).
Some part of the discussion around PLIST_SUB is not recorded in the PR,
but it is still related to
PR: ports/184706
Submitted by: amdmi3

Add new port graphics/osgearth, pkg-descr:
osgEarth is a C++ terrain rendering SDK. Just create a simple XML file,
point it at your imagery, elevation, and vector data, load it into your
favorite OpenSceneGraph application, and go! osgEarth supports all kinds
of data and comes with lots of examples to help you get up and running
quickly and easily.
WWW: http://osgearth.org/
PR: ports/184706
Submitted by: lbartoletti <coder@tuxfamily.org>